The availability of tissue culture cell lines has made it possible to study the regulation of proliferation and differentiation of specific hematopoietic cell types and the effects on these cells of known or suspected mediators and modulators. It was found previously in this laboratory that retinoic acid is a potent inducer of terminal differentiation of the human promyelocytic cell line, HL-60, and the human monoblast- and monocyte-like cell lines, U-937 and THP-1. In addition retinoic acid was found to induce differentiation of fresh cells in primary culture of patients with acute promyelocytic leukemia. While retinoic acid alone is capable of inducing terminal differentiation combinations of a physiological concentration of retinoic acid (10 nM) and either cAMP inducing agents (e.g., cAMP, prostaglandin E, or cholera toxin) or the conditioned medium from either activated T-cells or human leukemic T-cell lines were synergistic in inducing differentiation of HL-60. An activity called "differentiation inducing factor" or DIF has been purified to homogeneity from this conditioned medium. In addition to DIF, immune interferon-gamma has also been identified as having differentiation inducing activity. Studies with combinations of retinoic acid and recombinant interferongamma have yielded results that are identical to those obtained with retinoic acid and DIF. These results suggest that these combinations may have utility in the treatment of patients with some leukemias.